On 5/22/19 12:00 AM, Johannes Weiner wrote:
On Tue, May 21, 2019 at 05:40:42PM +0800, Yang Shi wrote:
Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
swapped out"), THP can be swapped out in a whole. But, nr_reclaimed
and some other vm counters still get inc'ed by one even though a whole
THP (512 pages) gets swapped out.
This doesn't make too much sense to memory reclaim. For example, direct
reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
could fulfill it. But, if nr_reclaimed is not increased correctly,
direct reclaim may just waste time to reclaim more pages,
SWAP_CLUSTER_MAX * 512 pages in worst case.
And, it may cause pgsteal_{kswapd|direct} is greater than
pgscan_{kswapd|direct}, like the below:
pgsteal_kswapd 122933
pgsteal_direct 26600225
pgscan_kswapd 174153
pgscan_direct 14678312
nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would
break some page reclaim logic, e.g.
vmpressure: this looks at the scanned/reclaimed ratio so it won't
change semantics as long as scanned & reclaimed are fixed in parallel.
compaction/reclaim: compaction wants a certain number of physical pages
freed up before going back to compacting.
kswapd priority raising: kswapd raises priority if we scan fewer pages
than the reclaim target (which itself is obviously expressed in order-0
pages). As a result, kswapd can falsely raise its aggressiveness even
when it's making great progress.
Other than nr_scanned and nr_reclaimed, some other counters, e.g.
pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed
too since they are user visible via cgroup, /proc/vmstat or trace
points, otherwise they would be underreported.
When isolating pages from LRUs, nr_taken has been accounted in base
page, but nr_scanned and nr_skipped are still accounted in THP. It
doesn't make too much sense too since this may cause trace point
underreport the numbers as well.
So accounting those counters in base page instead of accounting THP as
one page.
This change may result in lower steal/scan ratio in some cases since
THP may get split during page reclaim, then a part of tail pages get
reclaimed instead of the whole 512 pages, but nr_scanned is accounted
by 512, particularly for direct reclaim. But, this should be not a
significant issue.
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly
Switched back to use compound_order per Matthew
Fixed more counters per Johannes
v2: Added Shakeel's Reviewed-by
Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
mm/vmscan.c | 40 ++++++++++++++++++++++++++++------------
1 file changed, 28 insertions(+), 12 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b65bc50..1044834 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1250,7 +1250,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
case PAGEREF_ACTIVATE:
goto activate_locked;
case PAGEREF_KEEP:
- stat->nr_ref_keep++;
+ stat->nr_ref_keep += (1 << compound_order(page));
goto keep_locked;
case PAGEREF_RECLAIM:
case PAGEREF_RECLAIM_CLEAN:
@@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
goto activate_locked;
}
+ /*
+ * Account all tail pages when THP is added
+ * into swap cache successfully.
+ * The head page has been accounted at the
+ * first place.
+ */
+ if (PageTransHuge(page))
+ sc->nr_scanned +=
+ ((1 << compound_order(page)) -
+ 1);
+
may_enter_fs = 1;
Even if we don't split and reclaim the page, we should always account
the number of base pages in nr_scanned. Otherwise it's not clear what
nr_scanned means.
Sure.
/* Adding to swap updated mapping */
@@ -1315,7 +1326,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
if (unlikely(PageTransHuge(page)))
flags |= TTU_SPLIT_HUGE_PMD;
if (!try_to_unmap(page, flags)) {
- stat->nr_unmap_fail++;
+ stat->nr_unmap_fail +=
+ (1 << compound_order(page));
goto activate_locked;
}
}
@@ -1442,7 +1454,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
unlock_page(page);
free_it:
- nr_reclaimed++;
+ /*
+ * THP may get swapped out in a whole, need account
+ * all base pages.
+ */
+ nr_reclaimed += (1 << compound_order(page));
This expression is quite repetitive. Why not do
int nr_pages;
page = lru_to_page(page_list);
nr_pages = 1 << compound_order(page);
list_del(&page->lru);
if (!trylock_page(page))
...
at the head of the loop and add nr_pages to all these counters
instead?
Because it is unknown whether the THP will be swapped out as a whole or
will be split at this point. nr_scanned is fine, but nr_reclaimed is not.
@@ -1642,14 +1659,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
unsigned long skipped = 0;
- unsigned long scan, total_scan, nr_pages;
+ unsigned long scan, nr_pages;
LIST_HEAD(pages_skipped);
isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
scan = 0;
- for (total_scan = 0;
- scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src);
- total_scan++) {
+ while (scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src)) {
struct page *page;
Once you fixed the units, scan < nr_to_scan && nr_taken >= nr_to_scan
is an impossible condition. You should be able to write:
while (scan < nr_to_scan && !list_empty(src))
Yes.
Also, you need to keep total_scan. The trace point wants to know how
many pages were actually looked at, including the ones from ineligible
zones that were skipped over.
Aha, yes. The total_scan includes both scanned and skipped. Will fix in v4.
page = lru_to_page(src);
@@ -1659,7 +1674,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
if (page_zonenum(page) > sc->reclaim_idx) {
list_move(&page->lru, &pages_skipped);
- nr_skipped[page_zonenum(page)]++;
+ nr_skipped[page_zonenum(page)] +=
+ (1 << compound_order(page));
continue;
}
@@ -1669,7 +1685,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
* ineligible pages. This causes the VM to not reclaim any
* pages, triggering a premature OOM.
*/
- scan++;
+ scan += (1 << compound_order(page));
switch (__isolate_lru_page(page, mode)) {
case 0:
nr_pages = hpage_nr_pages(page);
Same here, you can calculate nr_pages at the top of the loop and use
it throughout.
Yes. Will fix in v4.
@@ -1707,9 +1723,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
skipped += nr_skipped[zid];
}
}
- *nr_scanned = total_scan;
+ *nr_scanned = scan;
trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
- total_scan, skipped, nr_taken, mode, lru);
+ scan, skipped, nr_taken, mode, lru);
update_lru_sizes(lruvec, lru, nr_zone_taken);
return nr_taken;
}
--
1.8.3.1
